In magnetic disk drives of the type in which a read/write head is moved inwardly and outwardly with respect to the disks in order to access various portions of the disk surface, it is usual to control the relative position of the read/write head by use of a servo circuit which utilizes information permanently encoded on the disk as an input to a circuit which compares this input with a command input to it by control circuitry. There are several different types of encoding schemes and associated circuitry found in the art. One involves the coding of one surface of a plurality of rigidly connected disks with servo positioning information only. Many or all of the tracks on the surface are entirely written with position information. In a second scheme, a blank disk is prewritten with position servo information at points spaced about the surface of the disk, interspersed by fields for the storage of data. The present invention is of this type.
It will be appreciated by those skilled in the art that among the objectives of a servo design are ease of manufacture of the circuitry and, in the case of a servo circuit in which position information is written permanently to portions of the disk, ease of this writing. Further, the scheme should be such that the design of the circuit is relatively easily implementable and does not require, for example, synchronization of detection mechanisms which require expensive or high tolerance components.